PCB News - Select PCB materials for RF high power applications

Although some high-power PCB applications are independent of the base station, most high-power PCB applications are related to the power amplifier of the base station. When designing such high-power RF applications, many aspects need to be considered. This article focuses on the application of base station power amplifier based on PCB, but the basic concepts discussed here are also applicable to other high-power applications.

Most high-power RF applications have thermal management problems, and some basic relationships need to be considered to do well in thermal management. For example, when the signal power is input into the circuit, the circuit with higher loss will produce higher heat; The other is related to frequency. The higher the frequency, the more heat will be generated. In addition, the increase of heat in any dielectric material will cause the change of dielectric material DK (dielectric constant), that is, dielectric constant temperature coefficient (tcdk). The change of loss leads to the change of circuit temperature, and the change of temperature leads to the change of DK. This DK change caused by tcdk will affect the performance of RF circuit and may affect system application.

For the heat loss relationship, a variety of different materials and corresponding PCB characteristics can be considered. Sometimes, when designers choose low loss materials for PCB Applications, they may only consider the dissipation factor (DF or loss tangent). DF is only the dielectric loss of the material, but there will be other losses in the circuit. The total circuit loss related to RF performance is insertion loss, which is composed of four losses, which is the sum of dielectric loss, conductor loss, radiation loss and leakage loss.

Circuits using very low loss materials with DF of 0.002 and very smooth copper foil will have relatively low insertion loss. However, if the same circuit with the same low loss material is still used, but electrolytic copper (ED) with large roughness is used instead of smooth copper, the insertion loss will increase significantly.

The surface roughness of copper foil will affect the conductor loss of the circuit. It should be clear that the surface roughness related to loss is the surface roughness of copper foil at the copper dielectric interface when processing laminates. In addition, if the medium used in the circuit is thin, the copper foil surface will be closer. At this time, the copper foil surface roughness will have a greater impact on the insertion loss than the relatively thick medium.

For high-power RF applications, thermal management is usually a common problem, and it is more advantageous to choose laminates with low DF and smooth copper foil. In addition, it is usually wise to choose laminates with high thermal conductivity. High thermal conductivity will help and effectively transfer heat from the circuit to the radiator.

The frequency heat relationship shows that assuming the same RF power at both frequencies, more heat will be generated as the frequency increases. Taking some thermal management experiments conducted by Rogers as examples, it is found that the heat rise of microstrip transmission line loaded with 80W RF power at 3.6 GHz frequency is about 50 ° C. When the same circuit is tested with 80W power at 6.1 GHz, the heat rise is about 80 ° C.

There are many reasons why the temperature increases with the increase of frequency. One reason is that the DF of the material will increase with the increase of frequency, which will lead to more dielectric loss and eventually increase the insertion loss and heat. Another problem is that the conductor loss increases with the increase of frequency. The increase of conductor loss is almost due to the decrease of skin depth with the increase of frequency. In addition, with the increase of frequency, the electric field will be more dense, and there will be greater power density in a given area of the circuit, which will also increase heat.

Finally, tcdk of materials has been mentioned many times in this article. It is an inherent property of materials whose DK changes with temperature, and it is a material property that is often ignored. For power amplifier circuits, 1 / 4 wavelength lines are used in matching networks, which are very sensitive to DK fluctuations. When DK changes greatly, 1 / 4 wavelength matching will shift, resulting in the change of the efficiency of the power amplifier, which is very undesirable.

In conclusion, when selecting high-frequency materials for high-power RF PCB applications, the materials should have low DF, relatively smooth copper foil, high thermal conductivity and low tcdk. Many trade-offs need to be made when considering these material properties and end use requirements. Therefore, it is always wise for designers to contact their material suppliers when selecting materials for high-power RF applications.